Implement a mechanism to disable rebase of local changes

[jleandroperez]

Simperium, by default, will rebase local changes when the object already exists remotely.

This can be a problem when dealing with seed data: local changes (the pristine object) will be rebased on top of the remote objects (potentially, with modifications).

Let's implement a mechanism to disable the rebase during initial objects sync.

• Document the new mechanism